Process for producing carboxylic acids

ABSTRACT

The present invention relates to a process for producing carboxylic acids. It relates more particularly to a process for producing carboxylic acids by oxidation of a hydrocarbon with oxygen or a gas containing oxygen, and even more particularly to the oxidation of cyclohexane to adipic acid. It relates to a process comprising a step consisting of hydrolysis of the esters formed during the oxidation step.

The present invention relates to a process for producing carboxylicacids.

It relates more particularly to a process for producing carboxylic acidsby oxidation of a hydrocarbon with oxygen or a gas containing oxygen,and even more particularly to the oxidation of cyclohexane to adipicacid.

Adipic acid is an important chemical compound used in many fields. Thus,adipic acid can be used as an additive in many products, both in thearea of foods and in concrete. However, one of the most important usesis its application as a monomer in the production of polymers, includingpolyurethanes and polyamides.

Several processes for producing adipic acid have been proposed. One ofthe most important, used industrially on a large scale, consists inoxidizing, in one or two step(s), cyclohexane to a mixture ofcyclohexanol/cyclohexanone with a gas containing oxygen or with oxygen.After extraction and purification of the cyclohexanol/cyclohexanonemixture, these compounds are oxidized in particular to adipic acid withnitric acid.

However, this process has a major drawback associated with the formationof nitrous vapour.

Many studies have been carried out in order to develop a process foroxidizing hydrocarbons with oxygen or a gas containing oxygen, thatmakes it possible to directly obtain carboxylic acids, mainly adipicacid.

These processes are described in particular in patents FR 2,761,984, FR2,791,667, FR 2,765,930 and U.S. Pat. No. 5,294,739.

Generally, the reaction is carried out in a solvent medium, the solventbeing a monocarboxylic acid such as acetic acid. Other solvents havebeen proposed, for instance the carboxylic acids that are lipophilic innature, described in Patent FR 2,806,079.

Many patents have described the operating conditions for this reactionand also the various steps for extracting the acids formed, purifyingthem and also recycling the non-oxidized hydrocarbon and the catalyst.

However, in this oxidation reaction, by-products form that can more orless substantially decrease the yield of the process. Among these,by-products containing an alcohol function, such as cyclohexanol, areparticularly harmful. In fact, they can react with the acids formed, soas to give esters and thus to greatly decrease the yield of recoveredcarboyxlic acids. According to the method of extraction and ofseparation of the acids, the esters are either recycled with thenon-oxidized hydrocarbon or are entrained with the recovered acids. Thepresence of esters in the reaction medium can result in a decrease inthe activity of the catalyst and especially in the formation of unwantedby-products resulting from the oxidation of these esters.

The problem of ester formation is all the more substantial since theoxidation reaction is less selective for acids.

One of the aims of the present invention is to propose a process forproducing carboxylic acids by oxidation of hydrocarbons using oxygen ora gas containing oxygen, in which the harmful effect of the estersformed is decreased.

To this effect, the invention proposes a process for producingcarboxylic acids by oxidation of a hydrocarbon with oxygen or a gascontaining oxygen, in the presence of a monocarboxylic acid-basedsolvent and of an oxidation catalyst, characterized in that the reactionmedium is treated so as to separate and extract the carboxylic acidsformed during the oxidation, and in that hydrolysis of the esters formedduring the oxidation reaction is carried out by treatment of thereaction medium, either before separation of said acids formed, or afterseparation of said acids formed by treatment of the organic phasederived from the reaction medium.

According to a preferential characteristic of the invention, thehydrolysis is carried out by addition to the medium to be treated of astrong acid and maintenance of said medium at a temperature of greaterthan 50° C., preferably of between 80° C. and 200° C.

The period of time for which the temperature is maintained depends onthe amount of esters to be hydrolysed, and is determined in a usualmanner by those skilled in the art during the setting of the operatingparameters of the process.

In order to perform the hydrolysis, water can be added to the medium tobe treated. However, this addition of water can be eliminated if theamount present in the medium or the water added with the strong acid issufficient.

As strong acid that is suitable for the invention, acids having a pKa ofless than 2 are preferred. By way of example, mention may be made ofsulphonic acid, sulphuric acid, nitric acid, hydrochloric acid,hydrobromic acid, orthophosphoric acid, triflic acid, or the like.

Generally, the amount of strong acid added is defined so as to have aconcentration by weight of less than approximately 10% relative to theweight of the reaction medium, preferably of between 0.1 and 10%,advantageously of between 0.1 and 4%.

In one embodiment of the invention, the strong acid is added in pureform, preferably in the form of a concentrated solution.

According to another embodiment of the invention, the strong acid isadded in a form that is carried on or attached to an inert material suchas a resin. This embodiment makes it possible to carry out thehydrolysis under ideal conditions and to be able to readily separate andrecover the strong acid. As acid compounds that are suitable for theinvention, mention may be made, by way of example, of sulphonic resins.However, any other equivalent resin or carrier for strong acid functionsmay be used, since the invention is not limited to the use of sulphonicresins.

In one embodiment of the invention, the extraction or separation, fromthe reaction medium, of the carboxylic acids formed is carried out bymeans of separation by settling out of the reaction medium into twophases, an aqueous phase and an organic phase. This separation bysettling out is obtained or promoted directly by cooling of the reactionmedium, when the concentration of water present in said medium issufficient to obtain the formation of two phases. When the amount ofwater present is not sufficient, an additional amount of water is addedto the reaction medium before carrying out the separation by settlingout, before or after cooling.

In another embodiment of the invention, the extraction of the carboxylicacids formed can be carried out by means of a liquid/liquid extractionby treatment of the reaction medium derived from the reactor with anextraction liquid.

The monocarboxylic acid solvent present in the reaction medium isadvantageously insoluble in the extraction liquid.

For the purpose of the patent, the products are considered to beinsoluble in the extraction liquid if their solubility in said liquid,measured at 90° C. and under atmospheric pressure, is less than or equalto 10% by weight relative to the liquid.

According to the invention, the hydrolysis of the esters isadvantageously carried out in the medium obtained after extraction ofthe carboxylic acids according to one of the embodiments describedabove, or by filtration if the carboxylic acid produced crystallizesafter cooling of the reaction medium.

However, the hydrolysis of the esters can, according to the invention,also be carried out in the reaction medium before the extraction or theseparation of the carboxylic acids formed. In this embodiment, the acidswill be extracted or recovered from the medium according to thetechniques described above, after the hydrolysis of the esters has beencarried out.

According to a preferred embodiment of the invention, the treatment witha strong acid is advantageously carried out after elimination, byevaporation or distillation, of the organic compounds exhibiting aboiling point of less than or equal to that of the alcohols and/orketones formed during the oxidation reaction. Thus, in the case of theoxidation of cyclohexane, the cyclohexane that has not reacted, and allthe organic compounds formed that have a boiling point less than that ofthe alcohol and of the ketone (cyclohexanol or cyclohexanone in the caseof the oxidation of cyclohexane), are separated from the medium bydistillation and, preferably, recycled in the oxidation step. Thealcohol and the ketone (cyclohexanol and cyclohexanone) are alsoseparated and recycled during this step. However, the treatment with astrong acid can also be carried out on the reaction medium before theseparation by distillation of the organic compounds described above.

According to another embodiment of the invention, the treatment with astrong acid in order to hydrolyse the esters is carried out afterelimination by distillation of the organic compounds exhibiting aboiling point less than or equal to that of the monocarboxylic solventused to carry out the oxidation, for instance aromatic carboxylic acids.This embodiment makes it possible to recycle the acid solvent with thehydrocarbon and the ketone and alcohol compounds in the oxidation step,before any bringing into contact with a strong acid.

According to another characteristic of the invention, the reactionmedium after hydrolysis of the esters is treated so as to, firstly,separate the alcohols formed and, secondly, recover the acids formedand, optionally, the monocarboxylic solvent. The separation of thealcohols formed, such as cyclohexanol in the case of the oxidation ofcyclohexane, is advantageously obtained by distillation. Themonocarboxylic solvent is recycled after separation of the acids formedduring the hydrolysis. This separation is advantageously obtained byextraction of said acids formed, with a solvent, such as water. It iscarried out either by addition of the extraction solvent and separationof the aqueous and organic phases by means of separation by settlingout, or in a liquid/liquid extraction process and device, the oxidationsolvent forming the organic phase.

The alcohol (cyclohexanol) separated is advantageously recycled at theoxidation step. The medium obtained after separation of the alcohols canbe treated so as to recover the carboxylic acids present, byprecipitation, crystallization or any other method.

Advantageously, the oxidation solvent present in the medium obtainedafter separation of the alcohol is separated from the dicarboxylic acidsor from the aqueous phase present by, in particular, the techniquesdescribed above. The oxidation solvent thus separated is recycled in theoxidation step after, advantageously, a purification, for example bydistillation. The aqueous phase containing the acids formed during thehydrolysis is, after extraction or separation of the oxidation solvent,advantageously mixed with the aqueous phase containing the diacidsformed during the oxidation, extracted on leaving the oxidation step, orobtained in the step of extraction of these diacids, or treated directlyso as to recover the acids present. This aqueous phase containing thediacids formed during the hydrolysis can also be mixed with theoxidation medium leaving the oxidation step before the extraction of thediacids formed.

The medium obtained after separation of the alcohols can also beintroduced into the step for liquid/liquid extraction of the carboxylicacids formed, in particular when the strong acid used to carry out thehydrolysis is in a carried form, and therefore can be readily separatedfrom the medium before it is introduced into the liquid/liquidextraction step.

In a particular embodiment of the invention, when the strong acid isnitric acid, the alcohol formed by the hydrolysis of the esters isoxidized to an acid in the hydrolysis medium. For this, an oxidationcatalyst can be added to the hydrolysis medium and the amount of strongacid added may be greater than 10% by weight. The medium obtainedcontaining acids is added directly in the step consisting ofcrystallization of the dicarboxylic acid, without any step of separationand recovery of the alcohol.

The reaction medium is generally obtained from the oxidation, withoxygen or a gas containing oxygen, of a hydrocarbon, more particularlyof an arylaliphatic cycloaliphatic hydrocarbon such as cyclohexane orcyclododecane. The oxidation reaction is generally carried out in thepresence of a solvent. The solvent may be very varied in nature insofaras it is not substantially oxidizable under the reaction conditions. Itmay in particular be chosen from polarprotic solvents and polaraproticsolvents. As polarprotic solvents, mention may be made, for example, ofcarboxylic acids having only primary or secondary hydrogen atoms, inparticular aliphatic acids having from 2 to 9 carbon atoms, such asacetic acid, perfluoroalkylcarboxylic acids such as trifluoroaceticacid, alcohols such as tert-butanol, halogenated hydrocarbons such asdichloromethane, and ketones such as acetone. As polaraprotic solvents,mention may be made, for example, of lower alkyl (=alkyl radical havingfrom 1 to 4 carbon atoms) esters of carboxylic acids, in particularaliphatic carboxylic acids having from 2 to 9 carbon atoms orperfluoroalkylcarboxylic acids, tetramethylenesulphone (or sulpholane)or acetonitrile, benzonitrile.

The solvent may also be chosen from carboxylic acids that are lipophilicin nature.

The expression “lipophilic acid compound that is suitable for theinvention” is intended to mean aromatic, aliphatic, arylaliphatic oralkylaromatic compounds comprising at least 6 carbon atoms, that maycomprise several acid functions and that have low water-solubility, i.e.a solubility of less than 10% by weight at ambient temperature (10°C.-30° C.).

As lipophilic organic compounds, mention may be made, for example, ofhexanoic acid, heptanoic acid, octanoic acid, 2-ethylhexanoic acid,nonanoic acid, decanoic acid, undecanoic acid, dodecanoic acid, stearicacid (octadecanoic acid) and their permethylated derivatives (completesubstitution of the hydrogens of the methylene groups with the methylgroup), 2-octadecylsuccinic acid, 3,5-ditert-butyl-benzoic acid,4-tert-butylbenzoic acid, 4-octylbenzoic acid, tert-butyl hydrogenorthophthalate, naphthenic or anthracenic acids substituted with alkylgroups, preferably of tert-butyl type, substituted derivatives ofphthalic acids, and fatty diacids such as dimer fatty acid. Mention mayalso be made of the acids belonging to the above families and bearingvarious electron-donating substituents (groups with a hetero atom of theO or N type) or electron-withdrawing substituents (halogens,sulphonimides, nitro groups, sulphonato groups, or the like).

In general, the solvent is chosen so as to advantageously obtain ahomogeneous phase under the temperature and pressure conditions at whichthe oxidation reaction is carried out. For this, it is advantageous forthe solubility of the solvent in the hydrocarbon or the reaction mediumto be at least greater than 2% by weight, and for at least onehomogeneous liquid phase comprising at least some of the hydrocarbons tobe oxidized and some of the solvent to be formed.

Advantageously, the solvent is chosen from those with lowwater-solubility, i.e. that have a water-solubility of less than 10% byweight at ambient temperature (10-30° C.).

However, it is possible, without departing from the context of theinvention, to use a solvent having a water-solubility that is greaterthan that indicated above, if the partition coefficient for thiscompound between the organic phase(s) of the reaction medium consistingessentially of the hydrocarbon to be oxidized, the oxidationintermediates and the nonorganic phase comprising the water formedduring the oxidation reaction makes it possible to obtain aconcentration of the solvent in said aqueous phase of less than 10% byweight.

The oxidation is in general carried out in the presence of a catalyst.This catalyst advantageously comprises a metal element chosen from thegroup comprising Cu, Ag, Au, Mg, Ca, Sr, Ba, Zn, Cd, Hg, Al, Sc, In, Tl,Y, Ga, Ti, Zr, Hf, Ge, Sn, Pb, V, Nb, Ta, Cr, Mo, W, Mn, Tc, Re, Fe, Ru,Os, Co, Rh, Ir, Ni, Pd, Pt, lanthanides such as Ce, and combinationsthereof.

These catalytic elements are used either in the form of compounds thatare advantageously at least partially soluble in the liquid oxidationmedium under the conditions under which the oxidation reaction iscarried out, or are carried by, absorbed onto or attached to an inertsupport such as silica or alumina, for example.

The catalyst is preferably, in particular under the conditions underwhich the oxidation reaction is carried out:

-   -   either soluble in the hydrocarbon to be oxidized,    -   or soluble in the lipophilic acid compound,    -   or soluble in the hydrocarbon/lipophilic acid compound mixture        forming a homogeneous liquid phase under the conditions under        which the reaction is carried out.        According to a preferred embodiment of the invention, the        catalyst used is soluble in one of these media at ambient        temperature or at the temperature for recycling of these media        in a further oxidation.

The term “soluble” is intended to mean that the catalyst is at leastpartially soluble in the medium under consideration.

In the case of a heterogeneous catalyst, the catalytically active metalelements are supported or incorporated in or into a microporous ormesoporous mineral matrix, or in or into a polymeric matrix, or are inthe form or organometallic complexes grafted onto an organic or mineralcarrier. The term “incorporated” is intended to mean that the metal isan element of the carrier or that the process is carried out withcomplexes that are sterically trapped in porous structures under theconditions of the oxidation.

In a preferred embodiment of the invention, the homogeneous orheterogeneous catalyst consists of salts or complexes of metals ofgroups IVb (the group of Ti), Vb (the group of V), VIb (the group ofCr), VIIb (the group of Mn), VIII (the group of Fe or Co or Ni) and Ib(the group of Cu) and cerium, alone or as a mixture. The preferredelements are, in particular, Mn and/or Co in combination with one ormore elements chosen from the group comprising Zr, Hf, Ce, Hf and Fe.The concentrations of metal in the liquid oxidation medium range between0.00001 and 5% (wt %), preferably between 0.001% and 2%.

Moreover, the concentration of solvent in the reaction medium isadvantageously determined so as to have a molar ratio of the number ofmolecules of solvent and the catalytic element metal number of between0.5 and 100 000, preferably between 1 and 5000.

The concentration of solvent in the liquid oxidation medium can varywithin broad limits. Thus, it can be between 1 and 99% by weightrelative to the total weight of liquid medium, more advantageously itcan be between 2 and 50% by weight of the liquid medium.

It is also possible, without nevertheless departing from the context ofthe invention, to use the solvent in combination with another compoundthat may in particular have the effect of improving the productivityand/or the selectivity of the reaction of oxidation to adipic acid, andin particular the solubilization of the oxygen.

As examples of such compounds, mention may in particular be made ofnitriles, hydroxyimide compounds, halogenated compounds, and moreadvantageously fluorinated compounds. As compounds that are moreparticularly suitable, mention may be made of nitriles such asacetonitrile or benzonitrile, imides belonging to the family describedin European Patent EP 0824962, and more particularlyN-hydroxysuccinimide (NHS) or N-hydroxyphthalimide (NHPI), halogenatedderivatives such as dichloromethane, and fluorinated compounds such as:

-   -   cyclic or acyclic, fluorinated or perfluorinated, aliphatic        hydrocarbons,    -   aromatic fluorinated hydrocarbons such as perfluorotoluene,        perfluoromethylcyclohexane, perfluoroheptane, perfluorooctane,        perfluorononane, perfluorodecaline, perfluoromethyldecaline,        α,α,α-trifluorotoluene or 1,3-bis-(trifluoromethyl)benzene,    -   perfluorinated or fluorinated esters such as alkyl        perfluorooctanoates or alkyl perfluorononanoates,    -   fluorinated or perfluorinated ketones such as perfluoroacetone,    -   fluorinated or perfluorinated alcohols such as perfluorohexanol,        perfluorooctanol, perfluorononanol, perfluorodecanol,        perfluoro-tert-butanol, perfluoroisopropanol or        1,1,1,3,3,3-hexafluoro-2-propanol,    -   fluorinated or perfluorinated nitriles such as        perfluoroacetonitrile,    -   fluorinated or perfluorinated acids such as        trifluoromethylbenzoic acids, pentafluorobenzoic acid,        perfluorohexanoic acid, perfluoroheptanoic acid,        perfluorooctanoic acid, perfluorononanoic acid or        perfluoroadipic acid,    -   fluorinated or perfluorinated halides such as        perfluoroiodooctane, or perfluorobromooctane,    -   fluorinated or perfluorinated amines such as        perfluorotripropylamine, perfluorotributylamine or        perfluorotripentylamine.

The invention applies more particularly to the oxidation ofcycloaliphatic compounds, such as cyclohexane or cyclododecane, to thecorresponding linear diacids, adipic acid or dodecanoic acid.

According to a preferred embodiment of the invention, it relates to thedirect oxidation of cyclohexane to adipic acid, with a gas containingoxygen, in a liquid medium and in the presence of a manganese catalyst.

The oxidation reaction is carried out at a temperature of between 50° C.and 200° C., preferably of between 70° C. and 180° C. It can be carriedout under atmospheric pressure. However, it is generally carried outunder a pressure so as to maintain the components of the reaction mediumin the liquid form. The pressure can be between 10 kPa (0.1 bar) and 20000 kPa (200 bar), preferably between 100 kPa (1 bar) and 10 000 kPa(100 bar).

The oxygen used may be in pure form or as a mixture with an inert gassuch as nitrogen or helium. Air more or less enriched with oxygen mayalso be used. The amount of oxygen supplied to the medium isadvantageously between 1 and 1000 mol per mole of compounds to beoxidized.

The oxidation process can be carried out continuously or according to abatch process. Advantageously, the liquid reaction medium that has leftthe reactor is treated according to known processes for, firstly,separating and recovering the diacid produced and, secondly, recyclingthe non-oxidized or partially oxidized organic compounds such ascyclohexane, cyclohexanol and/or cyclohexanone, the catalyst and theacid compound.

It is advantageous to also use a compound that initiates the oxidationreaction, for instance a ketone, an alcohol, an aldehyde or ahydroperoxide. Cyclohexanone, cyclohexanol and cyclohexyl hydroperoxide,which are reaction intermediates in the case of the oxidation ofcyclohexane, are all particularly indicated. In general, the initiatorrepresents from 0.01% to 20% by weight of the weight of the reactionmixture used, without these proportions having a critical value. Theinitiator is especially useful during the initiation of the oxidation.It can be introduced from the beginning of the reaction.

The oxidation can also be carried out in the presence of waterintroduced from the initial stage of the process.

As indicated above, the reaction mixture derived from the oxidation issubjected to various operations consisting in separating some of itsconstituents so as, for example, to allow the recycling thereof at thelevel of the oxidation and the recovery of the acids produced.

According to a first variant of the process, it is possible, first ofall, to subject the crude reaction mixture to cooling to a temperatureof 16° C. to 30° C., for example, which brings about the crystallizationof at least some of the acid formed. A medium is thus obtained,comprising a solid phase consisting essentially of acid, at least oneorganic liquid phase containing essentially the compound to be oxidizedthat is not reacted, possibly the acid compound and the oxidationintermediates (or several organic phases if the acid compound and thehydrocarbon are not completely miscible at low temperature) and anaqueous liquid phase containing essentially acid by-products of theoxidation and the water formed. The catalyst may be in one of theorganic phases if it is soluble in said phase, or in the lower aqueousphase.

After filtration or centrifugation of the solid, the organic and aqueousliquid phases constituting the filtrate or the centrifugate areseparated, if necessary, by settling out: the organic phase(s) may berecycled in a further oxidation reaction.

It may be advantageous, prior to the operation consisting ofcrystallization of the acid, to concentrate the reaction mixture.

According to a second variant of the process, the final crude reactionmixture can be withdrawn under hot conditions. The reaction mixture thenseparates by settling out into at least two liquid phases: one or moreorganic phases containing essentially the hydrocarbon that has notreacted, the acid compound and the oxidation intermediates and anaqueous liquid phase containing essentially the acids formed and thewater that has formed and/or been added. According to the solubility andthe nature of the catalyst, it may be present in the organic phase(s),recovered by solid/liquid separation before precipitation orcrystallization of the acid formed, in the case of a heterogeneouscatalyst, or, if it is soluble in the aqueous phase, extracted byliquid/liquid extraction, on resin, or electrodialysis.

As in the first variant, the liquid phases are separated by settlingout: the organic phase(s) can be recycled in a further oxidationreaction.

According to a third variant of the process of the invention, thereaction medium withdrawn from the reactor under hot conditions or aftercooling is introduced into a step consisting of liquid/liquid extractionof the carboxylic acids formed. The extraction liquid is generally thewater in which the acids formed are soluble; the organic compounds,hydrocarbons, alcohol, ketones and esters are insoluble, along with thesolvent used in the oxidation step.

As above, the catalyst may be in the organic fraction and will berecycled into the reaction medium. It may also be in the fractioncontaining the carboxylic acids, referred to, in the interests ofgreater simplicity, as the aqueous phase. The catalyst is recoveredaccording to the usual techniques listed above.

According to the present invention and a first embodiment thereof, thestep consisting of hydrolysis by addition of an acid and maintenance attemperature is carried out on the reaction medium before separation ofthe carboxylic acid or on the medium recovered after separation bysettling out or filtration of the crystallized acid.

According to a second embodiment of the invention, the hydrolysis of theesters is carried out by addition of an acid to the separated liquidorganic phase, before the recycling into the oxidation reactor, andoptionally water.

In these two embodiments, it may be advantageous to separate, prior tothe addition of the strong acid, the organic compounds such as thehydrocarbon that has not reacted, the alcohols and ketones formed andall the other products having a boiling point lower than said alcoholsand ketones, and also the monocarboxylic solvent, in an advantageousembodiment of the invention.

In these various embodiments, the recovered carboxylic acid can bepurified according to the usual techniques described in many documents,for example by crystallization and recrystallization from varioussolvents such as water, acetic acid or other organic solvents.Purification processes are in particular described in French Patents No.2,749,299 and 2,749,300.

Similarly, if the catalyst is not entirely recycled with the organicphase, and is partly or completely extracted with the aqueous phase, itwill be advantageously extracted from the aqueous phase by varioustechniques, such as liquid/liquid extraction, electrodialysis, ortreatment on ion exchange resin, for example.

The process of the invention makes it possible to limit the formation ofby-products that are in particular formed by the oxidation of the estersif the latter are not eliminated before the recycling. In addition, theelimination of the esters and the limitation of the formation ofby-products make it possible in particular to maintain the activity ofthe oxidation catalyst and to facilitate the extraction of the diacidsformed from the oxidation medium.

Other advantages and details of the invention will become apparent inview of the examples given below, only by way of indication.

EXAMPLES 1-A AND 1-B

Oxidation:

4 g of cobalt tetrahydrate, 357 g of acetic acid, 290 g of cyclohexaneand 3.6 g of cyclohexanone (initiator) are placed in a 1.5 l reactor.The mixture is stirred at 105° C. under a pressure of 20 bar and under acontinuous stream of gas containing nitrogen and oxygen. After 50 l ofoxygen have been consumed, a cyclohexane solution and a solution ofacetic acid containing 1.1% by mass of cobalt are injected continuously,the level in the reactor being maintained constant. The reaction mass isrecovered in a glass receptacle maintained at 70° C.

The reaction mixture continuously obtained is distilled under vacuum(120-145° C., 0.6 to 0.3 bar). Out of a mass of 2340 g involved in thedistillation, a distillation bottom product of 510 g is recovered. Thisbottom product constitutes “the reaction mixture after distillation ofthe light products” treated in Example 1-A below.

For Example 1-B, the reaction mixture used is the “reaction mixtureafter distillation of the light products” above that has undergonefurther elimination of the cyclohexanol/cyclohexanone (hereinafterreferred to as “olone”) compounds by azeotropic distillation in thepresence of water.

1-A Hydrolysis without Catalyst

The hydrolysis of the reaction mixture after distillation of the lightproducts (18.6 g) is carried out in the presence of H₂O (7.2 g), i.e. awater/ester molar ratio=77.5. The mixture is stirred at 115° C. for 18h, with continuous elimination of cyclohexanol using a “Dean-Stark”apparatus.

Under these conditions, 20% of the cyclohexyl esters are hydrolysed.

1-B Hydrolysis in the Presence of a Catalyst

The hydrolysis of the reaction mixture after distillation of the lightproducts and azeotropic distillation of the olone (15.3 g) is carriedout in the presence of H₂O (12.1 g, including 4.8 g of 2N nitricsolution). The mixture is stirred at 127° C. for 18 h, with continuouselimination of cyclohexanol using a “Dean-Stark” apparatus.

Under these conditions 90% of the cyclohexyl esters are hydrolysed.

EXAMPLES 2-A AND 2-B

In these examples, the “reaction mixture leaving the reactor” isobtained as follows:

522 g of cyclohexane, 55 g of tert-butylbenzoic acid and 6 g ofcyclohexanone (initiator) are placed in a 1.5 l reactor. Manganese andcobalt are added in respective amounts of 50 and 20 ppm by mass.

The mixture is stirred at 130° C., 20 bar, for 150 min under acontinuous stream of gas containing nitrogen and oxygen. After 63 l ofoxygen have been consumed, the stream of gas is stopped, the mixture iscooled, and the reactor is depressurized. A mass of 300 g of water isadded to the reactor with gentle stirring. The content of the reactor istransferred into a settler. After separation by settling out, two phasesare recovered: a lower phase, referred to as aqueous phase, whichcontains essentially the diacids produced and the catalytic metals, andan upper phase, referred to as organic phase, which contains essentiallycyclohexane, tert-butylbenzoic acid, cyclohexanone, cyclohexanol andother by-products of the reaction, including esters.

2-A Hydrolysis with Purolite NRW/160 Resins

The hydrolysis of the reaction mixture leaving the reactor (5.37 g) iscarried out in the presence of H₂O (5.14 g) and of a sulphonic resinsold by the company Aldrich under the name Purolite NRW160 (1.01 g). Themixture is stirred at 80° C. for 4 h.

Under these conditions, approximately 30% of the cyclohexyl esters arehydrolysed.

2-B Hydrolysis with Purolite NRW160 Resins

The hydrolysis of the reaction mixture leaving the reactor (5.28 g) iscarried out in the presence of H₂O (5.07 g) and of a Purolite NRW160sulphonic resin (5.03 g). The mixture is stirred at 100° C. for 4 h.

Under these conditions, approximately 70% of the cyclohexyl esters arehydrolysed.

EXAMPLE 3 Hydrolysis in the Presence of H₂SO₄

The hydrolysis of a reaction mixture after distillation of the lightproducts corresponding to that used in Example 1-A (2.6 g, including0.55 g of esters) is carried out in the presence of H₂O and H₂SO₄ (1 gof water, including 1% by mass of H₂SO₄). The mixture is stirred at 160°C. for 12 h.

Under these conditions, approximately 85% of the cyclohexyl esters arehydrolysed.

EXAMPLE 4 Hydrolysis with Amberlyst A31 Resin

The hydrolysis of a reaction mixture (10 g) corresponding to Example 2Aand after distillation of the compounds having a boiling point lowerthan or equal to that of the acid solvent tBBA is carried out in thepresence of 20 ml of Amberlyst A31 resin (sold by the company Rohm andHaas) and of 90 g of H₂O. The mixture is heated to 100° C., withcontinuous elimination of the cyclohexanol formed, using a Dean-Starkapparatus. The mixture is stirred at 100° C. for 4 h.

Under these conditions, approximately 95% of the esters are hydrolysedand the cyclohexanol formed is recovered.

EXAMPLE 5 Hydrolysis in the Presence of HNO₃

The hydrolysis of a reaction mixture (1.4 g) equivalent to Example 2A,after distillation of the light products and of the tBBA, is carried outin the presence of HNO₃ at 60% in water (10.1 g) and in the presence ofa catalyst consisting of a small amount of Cu(NO₃)₂, VO₃NH₄ and NaNO₂.

The mixture is stirred for 1 h at 70° C.

Under these conditions, the esters are completely hydrolysed and thecyclohexanol formed is completely converted to adipic acid.

1-21. (canceled)
 22. A process for producing carboxylic acids byoxidation of a hydrocarbon with oxygen or a gas containing oxygen withthe formation of esters in a reaction medium, in the presence of amonocarboxylic acid-based solvent and of an oxidation catalyst,comprising the steps of hydrolysing the esters formed by carrying out atreatment of the reaction medium before extraction of the carboxylicacids or a treatment of the organic phase derived from the reactionmedium after extraction of the carboxylic acids formed.
 23. A processaccording to claim 22, wherein the hydrolysis step is carried out byaddition to the medium to be treated of a strong acid and maintenance ofsaid medium at a temperature of greater than 50° C., optionally ofbetween 80° C. and 200° C.
 24. A process according to claim 23, whereinthe strong acid has a pKa of less than or equal to
 2. 25. A processaccording to claim 24, wherein the strong acid is carried on or attachedto an inert material such as a resin.
 26. A process according to claim25, wherein the resin is a sulphonic acid resin.
 27. A process accordingto claim 22, wherein the extraction of the carboxylic acids producedfrom the reaction medium is carried out by means of separation bysettling out.
 28. A process according to claim 22, wherein theextraction of the carboxylic acids produced from the reaction medium isobtained by liquid/liquid extraction.
 29. A process according to claim22, wherein the organic phase obtained after extraction of thecarboxylic acids and hydrolysis of the esters is recycled at theoxidation step.
 30. A process according to claim 22, wherein the organicphase recovered after separation of the diacids formed is subjected todistillation of the compounds having a boiling point less than or equalto that of the alcohol formed during the oxidation step, before thehydrolysis step.
 31. A process according to claim 22, wherein theorganic phase recovered after separation of the diacids formed issubjected to distillation of the compounds having a boiling point lessthan or equal to that of the acid solvent used in the oxidation step,before the hydrolysis step.
 32. A process according to claim 22, whereinthe acids formed during the hydrolysis step are extracted from themedium with a solvent for said acids.
 33. A process according to claim32, wherein the oxidation solvent present in the hydrolysis medium isextracted and purified before recycling at the oxidation step.
 34. Aprocess according to claim 32, wherein the acids recovered from thehydrolysis medium are mixed with the diacids extracted from theoxidation medium or in the oxidation medium before extraction of thediacids.
 35. A process according to claim 22, wherein the hydrocarbon isa cycloalkane.
 36. A process according to claim 35, wherein thecycloalkane is cyclohexane or cyclododecane.
 37. A process according toclaim 22, wherein the solvent is a monocarboxylic acid having from 1 to6 carbon atoms, or an acid lipophilic in nature, having from 7 to 20carbon atoms.
 38. A process according to claim 37, wherein thelipophilic acid is hexanoic acid, heptanoic acid, octanoic acid,2-ethylhexanoic acid, nonanoic acid, decanoic acid, undecanoic acid,dodecanoic acid, stearic acid (octadecanoic acid) and theirpermethylated derivatives, 2-octadecylsuccinic acid,3,5-ditert-butylbenzoic acid, 4-tert-butylbenzoic acid, 4-octylbenzoicacid, tert-butyl hydrogen orthophthalate, alkylnaphthenic acid,alkylanthracenic acid, a substituted derivative of phthalic acids, or afatty diacid.
 39. A process according to claim 38, wherein thelipophilic acid is a dimer fatty acid, a naphthenic acid substitutedwith tert-butyl groups, or an anthracenic acid substituted withtert-butyl groups.
 40. A process according to claim 22, wherein thecatalyst is a transition metal.
 41. A process according to claim 40,wherein the catalyst is based on manganese in combination with aco-catalyst which is cobalt, zirconium, cerium, hafnium or iron.
 42. Aprocess according to claim 22, wherein the polycarboxylic acid producedis adipic acid, succinic acid, glutaric acid, dodecanedioic acid or amixture thereof.